Using bleed air to supply outside air to a cabin

ABSTRACT

An environmental control system of an aircraft includes a compressing device including a compressor and a turbine configured to receive a flow of first medium sequentially. The compressing device additionally includes a second turbine configured to receive a flow of second medium distinct from the first medium. A dehumidification system is arranged in fluid communication with the turbine and a bypass valve is configured to divert the flow of the first medium output from the compressor around the turbine.

BACKGROUND

Embodiments of the disclosure relate to environmental control systems,and more specifically to an environmental control system of an aircraft.

In general, contemporary air condition systems are supplied a pressureat cruise that is approximately 30 psig to 35 psig. The trend in theaerospace industry today is towards systems with higher efficiency. Oneapproach to improve airplane efficiency is to eliminate the bleed airentirely and use electrical power to compress outside air. A secondapproach is to use lower engine pressure. The third approach is to usethe energy in the bleed air to compress outside air and bring it intothe cabin. Unfortunately, each of these approaches provides limitedefficiency with respect to engine fuel burn.

Early air conditioning systems commonly used on an aircraft, weretypically driven by pressurized air suppled from a turbo compressor.High pressure air drawn from an engine is delivered to a turbocompressor to compress outside air within the turbo compressor. Thecompressed outside air output from turbo compressor then passes througha series of heat exchangers, an air cycle machine, and a high pressurewater separator where the air is cooled and dehumidified. The resultingcool dry air is provided to the cabin, flight deck, and one or moreother systems of the aircraft.

BRIEF DESCRIPTION

According to one embodiment, an environmental control system of anaircraft includes a compressing device including a compressor and aturbine configured to receive a flow of first medium sequentially. Thecompressing device additionally includes a second turbine configured toreceive a flow of second medium distinct from the first medium. Adehumidification system is arranged in fluid communication with theturbine and a bypass valve is configured to divert the flow of the firstmedium output from the compressor around the turbine.

In addition to one or more of the features described above, or as analternative, in further embodiments the second medium is provided from ableed air source including at least one of an engine and an auxiliarypower unit of the aircraft.

In addition to one or more of the features described above, or as analternative, in further embodiments the first medium is fresh outsideair.

In addition to one or more of the features described above, or as analternative, in further embodiments the second medium output from thepower turbine is exhausted overboard.

In addition to one or more of the features described above, or as analternative, in further embodiments comprising a ram air circuit,wherein the second medium output from the power turbine is provided tothe ram air circuit.

In addition to one or more of the features described above, or as analternative, in further embodiments the power turbine is a dual entryturbine having a first inlet and a second inlet.

In addition to one or more of the features described above, or as analternative, in further embodiments at least a portion of the flow offirst medium output from the compressor is selectively provided to thefirst inlet and the flow of second medium is provided to the secondinlet.

In addition to one or more of the features described above, or as analternative, in further embodiments comprising a ram air circuitincluding a ram air duct having at least one heat exchanger positionedtherein.

In addition to one or more of the features described above, or as analternative, in further embodiments the at least one heat exchanger isconfigured to receive the flow of medium output from the compressor.

In addition to one or more of the features described above, or as analternative, in further embodiments the at least one heat exchangerincludes a first heat exchanger and a second heat exchanger, the firstheat exchanger is configured to receive the flow of medium output fromthe compressor, and the second heat exchanger is configured to receivethe flow of second medium.

In addition to one or more of the features described above, or as analternative, in further embodiments the second heat exchanger ispositioned upstream from the power turbine relative to the flow ofsecond medium.

In addition to one or more of the features described above, or as analternative, in further embodiments the environmental control system isoperable in a plurality of modes including a first mode and a secondmode.

In addition to one or more of the features described above, or as analternative, in further embodiments the environmental control system isoperable in the first mode when an ambient temperature is at or above adesign point of the environmental control system.

In addition to one or more of the features described above, or as analternative, in further embodiments the environmental control system isoperable in the second mode when an ambient temperature is below adesign point of the environmental control system.

In addition to one or more of the features described above, or as analternative, in further embodiments the bypass valve is in a closedposition during operation in the first mode and the bypass valve is inan open position during operation in the second mode

According to another embodiment, a method of operating an environmentalcontrol system of an aircraft includes providing a first medium to theenvironmental control system including a compressor and a turbine,wherein the first medium is provided to the compressor and the turbinesequentially and extracting work from a second medium provided to apower turbine operably coupled to the compressor to drive thecompressor. In a first mode of operation, the first medium to beprovided to a downstream load is output from the turbine and in a secondmode of operation, at least a portion of the first medium to be providedto a downstream load bypasses the turbine.

In addition to one or more of the features described above, or as analternative, in further embodiments the environmental control system istransformed from the first mode of operation to the second mode ofoperation by opening a bypass valve.

In addition to one or more of the features described above, or as analternative, in further embodiments in a third mode of operation, atleast a portion of the first medium output from the compressor isprovided to the power turbine.

In addition to one or more of the features described above, or as analternative, in further embodiments the environmental control system istransformed from the first mode of operation to the third mode ofoperation by opening a second bypass valve.

In addition to one or more of the features described above, or as analternative, in further embodiments providing the second medium to theenvironmental control system includes drawing bleed air from an engineof the aircraft.

Additional features and advantages are realized through the techniquesof the embodiments herein. Other embodiments are described in detailherein and are considered a part of the claims. For a betterunderstanding of the embodiments with the advantages and the features,refer to the description and to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter is particularly pointed out and distinctly claimed inthe claims at the conclusion of the specification. The forgoing andother features, and advantages thereof are apparent from the followingdetailed description taken in conjunction with the accompanying drawingsin which:

FIG. 1 is a schematic diagram of an environmental control systemaccording to an embodiment;

FIG. 2 is a schematic diagram of an environmental control systemaccording to an embodiment;

FIG. 3 is a schematic diagram of an environmental control systemaccording to another embodiment;

FIG. 4 is a schematic diagram of an environmental control systemaccording to yet another embodiment;

FIG. 5 is a schematic diagram of an environmental control systemaccording to yet another embodiment; and

FIG. 6 is a schematic diagram of an environmental control systemaccording to yet another embodiment.

DETAILED DESCRIPTION

A detailed description of one or more embodiments of the disclosedapparatus and method are presented herein by way of exemplification andnot limitation with reference to the FIGS.

Embodiments herein provide an environmental control system of anaircraft that receives multiple mediums from different sources and usesenergy from one or more of the mediums to operate the environmentalcontrol system and to provide cabin pressurization and cooling at a highfuel burn efficiency. The mediums described herein are generally typesof air; however, it should be understood that other mediums, such asgases, liquids, fluidized solids, or slurries are also contemplatedherein.

With reference now to FIG. 1, a schematic diagram of an environmentcontrol system (ECS) 20 is depicted according to a non-limitingembodiment. Although the environmental control system 20 is describedwith reference to an aircraft, alternative applications are also withinthe scope of the disclosure. As shown in the FIG., the system 20 canreceive a first medium A1 at a first inlet 22 and provide a conditionedform of the first medium A1 to a volume 24. In embodiments where theenvironmental control system 20 is used in an aircraft application, thefirst medium A1 is fresh or outside ambient air.

The system 20 receives a second medium A2 at an inlet 26. In oneembodiment, the second medium A2 is bleed air. As used herein, the term“bleed air” includes pressurized air originating from i.e. being “bled”from, an engine or auxiliary power unit of the aircraft. It shall beunderstood that one or more of the temperature, humidity, and pressureof the bleed air may vary based upon the compressor stage andrevolutions per minute of the engine or auxiliary power unit from whichthe air is drawn. For example, bleed air may be drawn from either a lowpressure compressor spool or a high pressure compressor spool of anengine, and bleed air drawn from the low pressure compressor spool willhave a relatively lower pressure than bleed air drawn from the highpressure compressor spool. In some embodiments, the system 20 isconfigured to extract work from the second medium A2. In this manner,the pressurized air A2 can be utilized by the system 20 to achievecertain operations.

The environmental control system 20 includes one or more heatexchangers. The one or more heat exchangers are devices built forefficient heat transfer from one medium to another. Examples of the typeof heat exchangers that may be used, include, but are not limited to,double pipe, shell and tube, plate, plate and shell, adiabatic shell,plate fin, pillow plate, and fluid heat exchangers. In an embodiment,the one or more heat exchangers may be located within the shell of a RAMair circuit (not shown), such that the one or more heat exchangers maybe referred to as “ram heat exchangers.” Within the one or more heatexchangers, a cooling fluid, such as outside air drawn in through ascoop for example, acts as a heat sink to cool a medium passing therethrough, for example the first medium A1 and/or the second medium A2. Inan embodiment, best shown in FIG. 1, the one or more heat exchangersincludes a first heat exchanger 30. In another embodiment, the one ormore heat exchangers includes the first heat exchanger 30 and a secondheat exchanger 32. The heat exchangers 30, 32 may be arranged in seriesrelative to the flow of cooling medium.

The system 20 additionally comprises at least one compressing device 40.In the illustrated, non-limiting embodiment, the compressing device 40of the system 20 is a mechanical device that includes components forperforming thermodynamic work on a medium (e.g., extracts work from orapplies work to the first medium A1 and/or the second medium A2 byraising and/or lowering pressure and by raising and/or loweringtemperature.) Examples of the compressing device 40 include an air cyclemachine, a three-wheel air cycle machine, a four-wheel air cyclemachine, etc. . . . .

As shown, the compressing device 40 includes a compressor 42, a turbine44, and a power turbine 46, operably coupled to each other via a shaft50 that is also is connected to a fan 48. The compressor 42 is amechanical device that raises a pressure of a medium provided theretoand can be driven by another mechanical device (e.g., a motor or amedium via a turbine). Examples of compressor types include centrifugal,diagonal or mixed-flow, axial-flow, reciprocating, ionic liquid piston,rotary screw, rotary vane, scroll, diaphragm, air bubble, etc. As shown,the compressor 42 is configured to receive and pressurize the secondmedium A2.

Each of the turbine 44 and the power turbine 46 is a mechanical devicethat expands a medium and extracts work therefrom (also referred to asextracting energy). In the compressing device 40, the power turbine 46drives the compressor 42 and the fan 48 via the shaft 50. In anembodiment, best shown in FIGS. 1 and 3, a single flow of a medium, suchas the second medium A2 for example, is provided to the power turbine46. However, in other embodiments, as shown in FIGS. 2 and 4, the powerturbine 46 can be a dual entry turbine that includes multiple inletfluid flow paths, such as an inner flow path and an outer flow path, toenable mixing of alternative medium flows within the turbine or at theexit of the power turbine 46. In an embodiment, the inner flow path is afirst diameter and the outer flow path is a second diameter. Further,the power turbine 46 may include a first nozzle configured to acceleratethe first medium for entry into a turbine impeller and a second nozzleconfigured to accelerate the second medium for entry into the turbineimpeller. The turbine impeller can be configured with a first gas pathconfigured to receive the first medium from the first nozzle and with asecond gas path configured to receive the second medium from the secondnozzle.

The fan 48 is a mechanical device that can force, via push or pullmethods, a medium (e.g., ram air) across the one or more heat exchangers30 and at a variable cooling to control temperatures.

The system 20 additionally includes at least one dehumidification system52. In the illustrated, non-limiting embodiment, the dehumidificationsystem 52 includes a condenser 54 and a water extractor 56. Thecondenser 54 is a particular type of heat exchanger and the waterextractor 56 is a mechanical device that removes water from a medium.The condenser 54 and the water extractor 56 are arranged to receive thefirst medium A1, and in some embodiments, both the first medium A1 andthe second medium A2. The configuration of the at least onedehumidification system 52 may vary. It should be understood that thedisclosed configuration of the dehumidification system is intended as anexample only, and embodiments including one or more additionalcomponents are also within the scope of the disclosure.

The elements of the system 20 are connected via valves, tubes, pipes,and the like. Valves (e.g., flow regulation device or mass flow valve)are devices that regulate, direct, and/or control a flow of a medium byopening, closing, or partially obstructing various passageways withinthe tubes, pipes, etc. of the system. Valves can be operated byactuators, such that flow rates of the medium in any portion of thesystem can be regulated to a desired value. For instance, a first valveV1 is configured to control a supply of the second medium A2 provided tothe system 20. A second valve V2 may be operable to allow a portion of amedium, such as the first medium A1, to bypass the turbine 44 of thecompression device 40. As a result, operation of the second valve V2 maybe used to add heat and to drive the compression device 40 duringfailure modes. In embodiments where the power turbine 46 is a dual entryturbine, a third valve V3 may be similarly operable to control a supplyof the first medium output from the compressor 42 to the power turbine46 as is illustrated in FIGS. 2 and 4.

The environmental control system of FIG. 1 may be operable in aplurality of modes based on a flight condition of the aircraft. Forexample, the environmental control system 20 may be operable in a firstmode when the ambient temperature is at or above a selected design pointof the system 20. In the first mode of operation, valve V1 is opened todraw a high pressure, hot second medium A2, such as bleed air, from ableed source, such as the turbine engine or the auxiliary power unit.This second medium A2 then enters the power turbine 46, such as via anozzle. The high pressure, high temperature second medium A2 is expandedacross the power turbine 46 and work extracted from the hot highpressure air. This extracted work drives the compressor 42 via shaft 50.This extracted work also drives the fan 48, which is used to move airthrough the heat exchanger 30, via a ram air duct (not shown). Thesecond medium output from the power turbine 46 may be exhaustedoverboard, into the ambient atmosphere

At the same time, a flow of cool, low pressure first medium A1 such asfresh outside air for example, is provided to an inlet of the compressor42. The act of compressing the fresh outside air, heats the freshoutside air. The compressed first medium A1 provided at the outlet ofthe compressor 42 (shown as CA1) then passes through an ozone converter60 before being provided to the heat exchanger 30. Within the heatexchanger 30, the compressed first medium CA1 is cooled via a flow ofram air. Embodiments where other components, such as an outflow heatexchanger (not shown) for example, are positioned directly downstreamfrom or upstream from the heat exchanger 30 are also within the scope ofthe disclosure.

The warm first medium A1 is then provided to at least a portion of thedehumidification system 52. As shown, the first medium A1 output fromthe heat exchanger 30 is provided sequentially to the condenser 54 andwater extractor 56 of the dehumidification system 52 where any freemoisture within the first medium A1 is condensed and removed, to producecool high-pressure air. This cool, high pressure first medium A1 thenenters the turbine 44 through an inlet or nozzle.

The cool, high pressure first medium A1 is expanded across the turbine44 and work is extracted therefrom. In combination with the workresulting from the power turbine 46, this extracted work drives thecompressor 42 and the fan 48, as previously described. The cold, forexample freezing, first medium A1 output from the turbine 44 enters thecondenser 54 to cool the warm first medium A1 leaving the heat exchanger30. The first medium A1 is then sent to one or more downstream loadsand/or locations of the aircraft. In an embodiment, the first medium A1output from the turbine 44 and the condenser 54 is mixed with anothermedium (not shown), such as recirculated air for example, such asprovided from the volume 22. The mixture of conditioned fresh air andrecirculated air may then be used to condition the volume 22, such asthe cabin and flight deck of an aircraft for example.

With continued reference to FIG. 1, the second mode of the environmentalcontrol system 20 is associated with operation of the system 20 atambient temperatures below a selected design point of the environmentalcontrol system. In the second mode of operation, the bypass valve V2 isopen, thereby allowing at least a portion of the first medium A1 outputfrom the compressor 42 to bypass the heat exchanger 30, the first passthrough the condenser 54, the water extractor 56, and the turbine 44. Insuch embodiments, at least a portion of the first medium A1 output fromthe compressor 42 is provided downstream from an outlet of the turbine44, before being provided to the condenser 54 and then delivered to oneor more downstream loads, such as to condition the volume 22.

The environmental control system 20 of FIG. 2 operates similarly to theenvironmental control system 20 of FIG. 1, except that in theillustrated, non-limiting embodiment, the power turbine 46 is a dualentry turbine. Accordingly, an additional conduit 62 connects the outletof the compressor 42 to a second inlet or nozzle of the power turbine46. A valve V3 is operable to selectively control a flow of the firstmedium A1 output from the compressor to the power turbine 46.

During operation of the system of FIG. 2 in a first mode, when theambient temperature is at or above a selected design point of the system20, valve V3 is at least partially open. Accordingly, the hot, mediumpressure fresh air output from the compressor 42 is split into a firstportion, A1 a and a second portion A1 b. The first portion of the firstmedium output from the compressor 42 is provided to power turbine 46 viathe conduit 62, and work is extracted therefrom. The first portion A1 aof the first medium and the second medium exhausted from the powerturbine 46 may be dumped overboard, or alternatively, into the ram aircircuit. The second portion A1 b of the first medium A1 output from thecompressor 42 is provided to the ozone converter 60, the heat exchanger30, and the remainder of the components of the system 20 as previouslydescribed with respect to FIG. 1.

With reference now to FIG. 3, the illustrated environmental controlsystem 20 is similar to the system of FIG. 1. However, in the embodimentillustrated in FIG. 3, the system 20 additionally includes a second heatexchanger 32 arranged downstream from the first heat exchanger 30relative to a flow of ram air within a ram air circuit defined by a ramair shell 34. In a first mode of operation, valve V1 is open and a highpressure, high temperature second medium A2, such as bleed air, isconfigured to pass through the second heat exchanger 32 before beingprovided to power turbine 46. Within the second heat exchanger, the highpressure, high temperature second medium A2 is cooled via a heatexchange relationship with the ram air flow. With the power turbine 46,the high pressure, warm temperature second medium A2 is expanded acrossthe power turbine 46 and work extracted therefore. This work drives thecompressor 42 and the fan 48 via shaft 50. Operation of the fan 48 movesair through the ram air circuit, across the heat exchangers 30, 32. Thesecond medium A2 output from the power turbine 46 may be exhaustedoverboard, into the ambient atmosphere, or alternatively, may be dumpedinto the ram air circuit, for example upstream from one or both of theheat exchangers 30, 32.

At the same time, a flow of cool, low pressure first medium A1, such asfresh outside air for example, is provided to an inlet of the compressor42. The act of compressing the fresh outside air, heats the freshoutside air. The compressed first medium A1 provided at the outlet ofthe compressor 42 may then pass through an ozone converter 60 beforebeing provided to the first heat exchanger 30. Within the heat exchanger30, the fast medium A1 is cooled via the flow of ram air.

The warm, moist first medium A1 is then provided to at least a portionof the dehumidification system 52. As shown, the first medium A1 outputfrom the first heat exchanger 30 is provided sequentially to thecondenser 54 and water extractor 56 of the dehumidification system 52where any free moisture within the first medium A1 is condensed andremoved, to produce cool high-pressure air. This cool, high pressurefirst medium A1 then enters the turbine 44 through an inlet or nozzle.

The cool, high pressure first medium A1 is expanded across the turbine44 and work is extracted therefrom. In combination with the workresulting from the power turbine 46, this extracted work drives thecompressor 42 and the fan 48, as previously described. The cold, forexample freezing, first medium A1 output from the turbine 44 enters thecondenser 52 to cool the warm first medium A1 leaving the heat exchanger30. The first medium A1 is then sent to one or more downstream loadsand/or locations of the aircraft. As previously described, in anembodiment, the first medium A1 output from the turbine 44 and thecondenser 54 is mixed with another medium, such as recirculated air,such as provided from the volume 22 for example. The mixture ofconditioned fresh air A1 and recirculated air (not shown) may then beused to condition the volume 22, such as the cabin and flight deck ofthe aircraft for example.

With continued reference to FIG. 3, a second mode of the environmentalcontrol system 20 may be associated with operation at ambienttemperatures below a selected design point of the environmental controlsystem 20. In the second mode of operation, the bypass valve V2 is open,thereby allowing at least a portion of the first medium A1 output fromthe compressor 42 to bypass the first heat exchanger 30, a first passthrough the condenser 54, the water extractor 56, and the turbine 44. Insuch embodiments, at least a portion of the first medium A1 output fromthe compressor 42 is provided downstream from an outlet of the turbine44, before being provided to the condenser 54 and then delivered to oneor more downstream loads, such as to condition the volume 22.

The environmental control system 20 of FIG. 4 is substantially identicalto the environmental control system 20 of FIG. 3, except that in theillustrated, non-limiting embodiment, the power turbine 46 is a dualentry turbine. Accordingly, an additional conduit 62 connects the outletof the compressor 42 to a second inlet or nozzle of the power turbine46. A valve V3 is operable to selectively control a flow of the firstmedium A1 output from the compressor to the power turbine 46.

During operation of the system of FIG. 4 in a first mode, when theambient temperature is at or above a selected design point of the system20, valve V3 is at least partially open. Accordingly, the hot, mediumpressure fresh air A1 output from the compressor 42 is split into afirst portion A1 a and a second portion A1 b. The first portion A1 a ofthe first medium A1 output from the compressor 42 is provided to powerturbine 46 via the conduit 62, and work is extracted therefrom. Thefirst portion A1 a of the first medium and the second medium A2exhausted from the power turbine 46 may be dumped overboard, oralternatively, into the ram air circuit as shown. The second portion A1b of the first medium A1 output from the compressor 42 is provided tothe ozone converter 60, the heat exchanger 30, and the remainder of thecomponents of the system 20 as previously described with respect toFIGS. 1-3.

The systems illustrated in FIGS. 5 and 6, are similar to the systems ofFIGS. 3 and 4, respectively. Although the condenser 54 is operable tocool the flow of first medium A1 such that the water therein may beremoved from the flow in the water extractor 56, in the systems shown inFIGS. 5 and 6, the condenser 54 of the dehumidification system 52 is nolonger arranged in fluid communication with the outlet of the turbine44. Rather, the condenser 54 is configured to receive fluid from theoutlet of the power turbine 46. By positioning the condenser in fluidcommunication with the power turbine 46, the flow of second medium A2used to cool the first medium A1 within the condenser is at atemperature above freezing. As a result, the potential for the formationof ice within the condenser 54 is generally reduced. In addition, thepressure of the air provided to the volume 24 will remain generallyequal to the pressure of the air output from the turbine 44. Afterpassing through this condenser 54, the second medium A2 may be exhaustedoverboard, or alternatively, into the ram air circuit, upstream from theram air heat exchangers 30, 32.

Aspects of the embodiments are described herein with reference toflowchart illustrations, schematics, and/or block diagrams of methods,apparatus, and/or systems according to embodiments. Further, thedescriptions of the various embodiments have been presented for purposesof illustration, but are not intended to be exhaustive or limited to theembodiments disclosed. Many modifications and variations will beapparent to those of ordinary skill in the art without departing fromthe scope and spirit of the described embodiments. The terminology usedherein was chosen to best explain the principles of the embodiments, thepractical application or technical improvement over technologies foundin the marketplace, or to enable others of ordinary skill in the art tounderstand the embodiments disclosed herein.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting. As used herein, thesingular forms “a”, “an” and “the” are intended to include the pluralforms as well, unless the context clearly indicates otherwise. It willbe further understood that the terms “comprises” and/or “comprising.”when used in this specification, specify the presence of statedfeatures, integers, steps, operations, elements, and/or components, butdo not preclude the presence or addition of one more other features,integers, steps, operations, element components, and/or groups thereof.

The flow diagrams depicted herein are just one example. There may bemany variations to this diagram or the steps (or operations) describedtherein without departing from the spirit of embodiments herein. Forinstance, the steps may be performed in a differing order or steps maybe added, deleted or modified. All of these variations are considered apart of the claims.

While the preferred embodiment has been described, it will be understoodthat those skilled in the art, both now and in the future, may makevarious improvements and enhancements which fall within the scope of theclaims which follow. These claims should be construed to maintain theproper protection.

What is claimed is:
 1. An environmental control system of an aircraftcomprising: a compressing device including a compressor and a turbineconfigured to receive a flow of first medium sequentially, and a secondturbine configured to receive a flow of second medium, the second mediumbeing distinct from the first medium; a dehumidification system in fluidcommunication with the turbine; and a bypass valve configured to divertthe flow of the first medium output from the compressor around theturbine.
 2. The environmental control system of claim 1, wherein thesecond medium is provided from a bleed air source including at least oneof an engine and an auxiliary power unit of the aircraft.
 3. Theenvironmental control system of claim 1, wherein the first medium isfresh outside air.
 4. The environmental control system of claim 1,wherein the second medium output from the power turbine is exhaustedoverboard.
 5. The environmental control system of claim 1, furthercomprising a ram air circuit, wherein the second medium output from thepower turbine is provided to the ram air circuit.
 6. The environmentalcontrol system of claim 1, wherein the power turbine is a dual entryturbine having a first inlet and a second inlet.
 7. The environmentalcontrol system of claim 6, wherein at least a portion of the flow offirst medium output from the compressor is selectively provided to thefirst inlet and the flow of second medium is provided to the secondinlet.
 8. The environmental control system of claim 1, furthercomprising a ram air circuit including a ram air duct having at leastone heat exchanger positioned therein.
 9. The environmental controlsystem of claim 8, wherein the at least one heat exchanger is configuredto receive the flow of medium output from the compressor.
 10. Theenvironmental control system of claim 8, wherein the at least one heatexchanger includes a first heat exchanger and a second heat exchanger,the first heat exchanger is configured to receive the flow of mediumoutput from the compressor, and the second heat exchanger is configuredto receive the flow of second medium.
 11. The environmental controlsystem of claim 10, wherein the second heat exchanger is positionedupstream from the power turbine relative to the flow of second medium.12. The environmental control system of claim 1, wherein theenvironmental control system is operable in a plurality of modesincluding a first mode and a second mode.
 13. The environmental controlsystem of claim 12, wherein the environmental control system is operablein the first mode when an ambient temperature is at or above a designpoint of the environmental control system.
 14. The environmental controlsystem of claim 12, wherein the environmental control system is operablein the second mode when an ambient temperature is below a design pointof the environmental control system.
 15. The environmental controlsystem of claim 12, wherein the bypass valve is in a closed positionduring operation in the first mode and the bypass valve is in an openposition during operation in the second mode.
 16. A method of operatingan environmental control system of an aircraft comprising: providing afirst medium to the environmental control system including a compressorand a turbine, wherein the first medium is provided to the compressorand the turbine sequentially; and extracting work from a second mediumprovided to a power turbine operably coupled to the compressor to drivethe compressor; wherein in a first mode of operation, the first mediumto be provided to a downstream load is output from the turbine and in asecond mode of operation, at least a portion of the first medium to beprovided to a downstream load bypasses the turbine.
 17. The method ofclaim 16, wherein the environmental control system is transformed fromthe first mode of operation to the second mode of operation by opening abypass valve.
 18. The method of claim 17, wherein in a third mode ofoperation, at least a portion of the first medium output from thecompressor is provided to the power turbine.
 19. The method of claim 18,wherein the environmental control system is transformed from the firstmode of operation to the third mode of operation by opening a secondbypass valve.
 20. The method of claim 16, wherein providing the secondmedium to the environmental control system includes drawing bleed airfrom an engine of the aircraft.